The present invention relates to chemical milling of metals and is particularly related to a process for recovering the etchant used in treating aluminum and aluminum alloys.
Conventionally chem-milling of aluminum in the aircraft industry involves reacting the part to be milled with a solution which contains sodium hydroxide (NaOH) as the basis of the etchant. One major problem with chem-milling compositions of this type, which have sodium hydroxide as their base is that the solution does not produce an acceptable part when the dissolved metal reaches about 60-70 g/l of solution. This is caused by interference of the reaction by-products with the chem-milling reaction.
The typical aluminum etchant reaction is: EQU 2Al+2NaOH+2H.sub.2 O.fwdarw.2NaAlO.sub.2 +3H.sub.2 .uparw.
The amount of NaAlO.sub.2 in the etching solution is what eventually causes the chem-milling solution in the tank to become deficient in regards to the finish imparted to the surface of the part being milled.
Aluminum chemical milling has, since its inception, been a batch process. That is, starting with fresh solution, metal is dissolved to a total ionic content of between 60-70 g/l and then no longer provides good chem-mill characteristics and has to be discarded. At this time large amounts of caustic are lost. Costs are also incurred in the hauling and disposal of such etchants. Numerous attempts to recover both dissolved aluminum and sodium hydroxide based upon precipitation techniques have been tried. This recovery is highly desirable and some techniques have been reported in the patent literature.
U.S. Pat. No. 2,975,041 to Holman alleges an etching process for aluminum which also provides for at least partial regeneration of spent caustic etching solutions. This process comprises contacting aluminum with an alkaline solution to etch or mill the metal. In this conventional operation, the free alkalinity of the etching solution is reduced and the aluminum values in solution are increased. A principal teaching of this patent is to cool the spent etching solution so as to precipitate hydrated alumina therefrom. If an insubstantial amount of solid phase hydrate alumina is formed to effect precipitation, a sufficient amount of solid phase hydrated alumina is added to the spent etching solution to cause a thorough contact between the solution and the dispersed solid alumina hydrate. The resulting cooled solution is allowed to stand for an extended period of time whereby hydrated alumina precipitates therefrom and the supernatant solution is separated by settling of the solids and decanting of the clear solution. According to this patent, "the solution shows an increase in the free alkali and a decrease in the amount of the aluminate in solution." This process for regenerating spent aluminum etching solution suffers from the disadvantages that the solution must be cooled, many times undesirable solids must be added to the etching bath, lengthy storage times must be utilized and there is no real control of the degree and quality of the regenerated bath.
Weissenberg U.S. Pat. No. 3,712,838 discloses a process of regenerating spent caustic liquor involving contacting the hot spent caustic bath with calcium oxide to precipitate aluminum values present in the spent liquor and regenerate hydroxyl ions. The precipitate is removed from the regenerated solution. The calcium oxide reacts with the aluminate values in the spent caustic solution to provide a substantially insoluble, readily removable solid, calcium aluminate precipitate. In addition to aluminate removal, Weissenberg also states his process regenerates the hydroxyl ion concentration in the caustic solution due to the calcium oxide reacting with the soluble anions in the spent solution to form an insoluble calcium salt and free hydroxyl ions. These anions are introduced into the solution during the etching operation. The solids are removed from the solution and the regenerated caustic values ordinarily returned to the etching process for reuse.
Both of the foregoing processes are theoretically of interest, but cannot be used on a continuous production basis without severe chemical milling defects, i.e., chattered radii, poor surface finish, and poor chemical milling lines.
It is believed that these production problems are caused by etch solution additives, alloying elements in the etched material, and organic and inorganic contaminants deriving from sources, such as, maskants and resists, used in the chemical milling procedure.
We have discovered that by adding calcium oxide and a barium salt to a depleted chemical milling solution, pure sodium hydroxide and unreacted sulfides can be removed for reuse in a chemical milling process on a substantially continuous basis.
We have developed an aluminum etchant rejuvenation system which precipitates accumulated dissolved aluminum, silicates and organic contaminants. The contaminants occur due to chem-mill "maskant" exposure in high caustic environments. In the chemical reactions associated with this process, caustic (NaOH) is generated which compensates for losses during metal precipitation. The process uses lime and barium salts, i.e., BaCl.sub.2, Ba(NO.sub.3).sub.2, etc.
The other materials are removed by a proper separation technique, i.e., centrifugation, filtration, or the like as precipitates or sol-precipitates. These include the dissolved aluminum in the form of Ca(AlO.sub.2).sub.2 and the silicate compounds, other metallic salts (from the alloying compounds) and the organics in the form of barium compounds as sol-precipitates.
Accordingly, it is a principal object of the present invention to provide a novel method for regenerating spent caustic aluminum etching solutions.
It is another object of the present invention to provide a process for regenerating spent caustic etching solutions wherein the solution is treated at its normal operating conditions.
Still another object is to provide a continuous process for chemically milling aluminum in which the spent etchant is recovered and the usable caustic rejuvenated on a continuous basis.
Other objects and advantages of the present invention include saving chemical cost by 70-80% recovery of unused sodium hydroxide; providing a continuous process and operating over a much narrower range of dissolved metal content, thus proving improved etch rate control; providing more uniform tank conditions and alleviating surfaced fillet variations on parts caused by the wide operating range of a batch operation; and saving in waste collection, hauling and storage costs.
These and other objects and advantages will become apparent hereinafter.